1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic type or electrostatic recording type and, in particular, an image forming apparatus such as a copying machine, a printer and a fax machine.
2. Description of the Related Art
In an image forming apparatus of electrophotographic type such as a printer and a copying machine that forms an image, a light image corresponding to an original is exposed on an electrostatic latent image bearing member such as a charged photosensitive member to form an electrostatic latent image, a developed toner image is formed on the electrostatic latent image by use of a developing means and the developed toner image is transferred on a recording material. Subsequently, in a fusing (fixing) device of heated roller type, a copied image corresponding to the original is formed by heating and pinching the recording material holding the toner image under pressure for fixation in a contact part (nipped part) between a fusing roller and a pressure roller.
In such fusing device, when the number of image outputs per unit time (hereinafter referred to as “productivity”) is increased, heat quantity taken from the fusing roller by the recording material increases in proportion to the increase in productivity. Therefore, as the productivity of the image forming apparatus is increased, falling of the temperature of the fusing roller becomes substantive and imperfect fusing occurs when the fusing roller temperature falls below the temperature at which fusing property can be maintained (hereinafter referred to as “fusing minimum temperature”).
A possible method for preventing this imperfect fusing is to increase electric power of heat source such as a halogen heater for heating the fusing roller, thereby to restrain falling of the fusing roller temperature. However, in the condition where temperature is easy to fall, for example, under low temperature or just after the time when the main unit is turned on, it is very difficult within general commercial power supply to feed the power enough to maintain the fusing roller temperature at the fusing minimum temperature or higher to the above-mentioned power source.
Accordingly, to avoid these problems, the control in which image formation is discontinued when the surface temperature of the fusing roller is detected and the detected temperature falls below a predetermined temperature, and is restarted when the detected temperature returns to the predetermined temperature is devised. The predetermined temperature is referred to as “stop temperature” and the control is referred to as “stop control”.
The control in which an interval of image formation is increased when the surface temperature of the fusing roller is detected and the detected temperature falls below a predetermined temperature, thereby to lower the productivity and restrain falling of the surface temperature of the fusing roller is also devised. The predetermined temperature is referred to as “down temperature” and the control is referred to as “down control”.
According to these two types of control, even in the condition where temperature is easy to fall such as low temperature surrounding, since the fusing roller temperature can be kept at the fusing minimum temperature or higher, the fusing property can be advantageously ensured.
For this reason, in the black-and-white copying machines and printers having high image productivity, the above-mentioned stop control and down control are performed. Further, according to the control, both high image productivity and fusing property can be realized within general commercial power supply.
Image productivity qualitatively represents the number of recording materials on which an image is formed per unit time and high productivity represents that the number of recording materials on which an image is formed per unit time is large.
On the other hand, the full-color image forming apparatus is generally configured so as to execute single-color mode of forming a single-color image by using one of magenta, cyan, yellow and black toners and full-color mode of forming a full-color image by mixing four colors of magenta, cyan, yellow and black toners. The user can select either of these modes as necessary.
In the full-color mode, in contrast to the single-color mode, since an image is formed by mixing four colors of toner, the maximum amount of toner held on the recording material becomes larger. Therefore, it is devised that the temperature at the fusing by the fusing roller in the full-color mode is higher than that in the single-color mode (for example, Unexamined Patent Publication No. 10-039673).
Although the high image productivity is desired also in the full-color image forming apparatus as in the black-and-white image forming apparatus, adoption of the above-mentioned stop control and down control causes the following problem.
That is, when the above-mentioned stop temperature and down temperature in the single-color mode and the full-color mode is uniformly set at the temperature at which fusing of the full-color image is ensured, despite that the fusing roller temperature falls within the range of temperatures at which the single-color image can be fixed, the operation proceeds to the stop control or down control during the job of forming the single-color image continuously, thereby to result in image productivity of single-color mode slowdown.
On the other hand, when the above-mentioned stop temperature and down temperature in the single-color mode and the full-color mode is uniformly set at the temperature at which fusing of the single-color image is ensured, imperfect fusing offset occurs due to low temperature offset and so on during the job of forming the full-color image continuously, in the event that the temperature falls below the temperature at which fusing of the full-color image is ensured.
As described above, in the conventional full-color image forming apparatus, it is difficult to realize image productivity and fusing property simultaneously.